This invention generally relates to welding apparatus and methods and more particularly relates to a system and method for laser welding an inner surface of a tubular member, which tubular member may be a repair sleeve disposed in a nuclear heat exchanger heat transfer tube.
Although laser welding apparatus and methods are known, it has been observed that such apparatus and methods have a number of operational problems associated with them that make these apparatus and methods less than completely satisfactory for welding an inner surface of a tubular member, which tubular member may be a repair sleeve disposed in a nuclear heat exchanger heat transfer tube. However, before these problems can be appreciated, some background is desirable as to the structure and operation of a typical nuclear heat exchanger.
In a typical nuclear heat exchanger or steam generator, a heated and radioactive primary fluid flows through a plurality of U-shaped tubes, each of the tubes having a fluid inlet and a fluid outlet end. The inlet and outlet ends of the tubes are received through holes in a tubesheet disposed in the heat exchanger. The heat exchanger defines an inlet plenum chamber below the tubesheet, which inlet plenum chamber is in communication with the inlet ends of the tubes. The heat exchanger also defines an outlet plenum chamber below the tubesheet and isolated from the inlet plenum chamber, the outlet plenum chamber being in communication with the outlet ends of the tubes. During operation of the heat exchanger, a heated and radioactive primary fluid flows into the inlet plenum chamber and enters the inlet ends of the tubes to flow through the tubes. The primary fluid then flows through the outlet ends of the tubes and into the outlet plenum chamber. The primary fluid next flows out the outlet plenum chamber to exit the heat exchanger. A nonradioactive secondary fluid having a temperature less than the primary fluid simultaneously surrounds the exterior surfaces of the tubes above the tubesheet as the primary fluid flows through the tubes. As the primary fluid flows through the tubes, it gives-up its heat to the secondary fluid surrounding the exterior surfaces of the tubes to produce steam that is used to generate electricity in a manner well known in the art.
Because the primary fluid is radioactive, the heat exchanger is designed such that the radioactive primary fluid flowing through the tubes does not commingle with and radioactively contaminate the nonradioactive secondary fluid surrounding the exterior surfaces of the tubes. Therefore, the tubes are designed to be leak-tight so that the radioactive primary fluid remains separated from the nonradioactive secondary fluid to avoid commingling the primary fluid with the secondary fluid.
Occasionally, due to tube wall intergranular cracking caused by stress and corrosion during operation of the heat exchanger, the heat exchanger tubes may degrade and thus may not remain leak-tight. Therefore, it is desirable to inspect the tubes to detect any tubes that may have experienced such stress corrosion cracking or degradation. This inspection is typically performed by inserting an inspection probe, such as an eddy current probe or an ultrasonic probe, into the tube and moving the probe along the interior surface of the tube. If the inspection probe indicates that stress corrosion cracking is present or eminent at a particular location, then the tube is "sleeved" at that location. When sleeving is performed, a tubular sleeve is inserted into the tube, so as to cover the degraded portion of the tube, and affixed thereto typically by expanding the sleeve into intimate engagement with the tube. The sleeve is expanded into engagement with the tube by any of several means known in the art, such as by hydraulic expansion, hard rolling or by other expansion techniques. In this manner, the sleeved tube remains in service although degraded.
However, the elastic properties of the sleeve may cause the sleeve to experience "spring back" after expansion. This phenomenon of "spring back" will in turn cause a relatively small gap to exist at the sleeve-to-tube interface. Such a gap is undesirable because the gap defines a flow path between the sleeve and the tube, which flow path may allow the primary fluid to undesirably commingle with the secondary fluid. Therefore, it is desirable to fuse the sleeve to the tube by forming, for example, two spaced-apart weldments circumscribing the sleeve in order to seal the gap and the flow path defined thereby. Relatively small sized gaps are sealed by braze welding. However, laser welding is preferred to seal gaps of relatively larger size (e.g., gaps of up to approximately 0.008 inch). Moreover, brazing requires precleaning, such as by honing the surface of the sleeve to remove any surface oxidation in order to provide uniform braze flow. Laser welding, on the other hand, obviates the need for surface precleaning.
An Nd:YAG laser is commonly used for fusing the sleeve to the tube by means of a laser beam. However, when an Nd:YAG laser is used to generate the laser beam, the laser beam may have a relatively large divergent angle of approximately 20-30 mrad, thereby causing difficulty in precisely confining the laser beam to the relatively small predetermined area of the sleeve to be welded. Therefore, a problem in the art is to provide a laser beam, such as a laser beam generated by an Nd:YAG laser, that is capable of being precisely confined.
Moreover, the laser beam generated by the Nd:YAG laser generally has less than fully acceptable spatial coherency, thereby resulting in a laser beam not having the desired high power density as it welds the sleeve. Less than desirable power density may result in a weldment that does not completely fuse the sleeve to the tube. Therefore, another problem in the art is to provide a laser beam, such as a laser beam generated by an Nd:YAG laser, that has a high power density.
Moreover, the typical laser welding apparatus uses at least one mirror to reflect the laser beam onto the sleeve and at least one lens to focus the laser beam onto the mirror. However, the mirror and lens are susceptible to fouling by weld spatter and by microscopic debris entrained in a weld plume produced during the welding process. In addition, such weld spatter and weld plume debris obstruct the laser beam as the laser beam travels through the lens to the mirror. Such fouling of the mirror and lens is undesirable because such fouling will shorten the useful life of the mirror and lens and will interfere with the ability of the welding apparatus to satisfactorily perform its welding function. Although a shielding gas, such as oxygen, hydrogen or the like, is forced over the mirror and lens in prior art laser welding devices to mitigate the effects of fouling and beam obstruction, it has been observed by applicant that such forced gas flow may be less than totally effective. Therefore, yet another problem in the art is to perform the laser welding process in such a manner that the mirror and lens are less susceptible to fouling and so that the laser beam is not as easily obstructed.
A laser welding head for welding a sleeve within a tube by fusing the interface between the sleeve and tube with a laser beam is disclosed in U.S. Pat. No. 4,694,137 issued Sep. 15, 1987 in the name of Phillip J. Hawkins, et al. titled "Laser Welding Head For Sleeve-To-Tube Welding" and assigned to the assignee of the present invention. However, this patent does not appear to disclose a solution to the problem of providing a laser beam that is precisely confined and that has the desired high power density. In addition, this patent does not appear to disclose a laser welding apparatus that satisfactorily reduces fouling of the mirror and lens during the welding process.
A system and method for laser welding the inner surface of a metallic tube is disclosed in U.S. Pat. No. 5,066,846 issued Nov. 19, 1991 in the name of William E. Pirl titled "System And Method For Laser Welding The Inner Surface Of Heat Exchanger Tubes" and assigned to the assignee of the present invention. However, this patent does not appear to disclose a solution to the problem of providing a laser beam that is precisely confined and that has the desired high power density. In addition, this patent does not appear to disclose a laser welding apparatus that satisfactorily reduces fouling of the mirror and lens during the welding process.
Therefore, what is needed are a system and method for suitably laser welding an inner surface of a tubular member, which tubular member may be a repair sleeve disposed in a nuclear heat exchanger heat transfer tube.